Add support for python 3.11

[parthea]

The following failure occurs with python 3.11 . See build log here.

I managed to get tests in python 3.11 to pass by changing CoroutineMock to AsyncMock but that didn't work for python 3.7. Python 3.7 will be end of life in the python community in 4 months so perhaps we could add python 3.11 support and skip python 3.7 tests. https://endoflife.date/python.

https://github.com/googleapis/python-pubsublite/compare/python-311-testing?expand=1

==================================== ERRORS ====================================
_ ERROR collecting tests/unit/pubsublite/cloudpubsub/internal/ack_set_tracker_impl_test.py _
tests/unit/pubsublite/cloudpubsub/internal/ack_set_tracker_impl_test.py:15: in <module>
    from asynctest.mock import MagicMock, call
.nox/unit-3-11/lib/python3.11/site-packages/asynctest/__init__.py:[22](https://github.com/googleapis/python-pubsublite/actions/runs/3850969666/jobs/6561686115#step:5:23): in <module>
    from .case import *
.nox/unit-3-11/lib/python3.11/site-packages/asynctest/case.py:54: in <module>
    import asynctest.selector
.nox/unit-3-11/lib/python3.11/site-packages/asynctest/selector.py:29: in <module>
    from . import mock
.nox/unit-3-11/lib/python3.11/site-packages/asynctest/mock.py:4[28](https://github.com/googleapis/python-pubsublite/actions/runs/3850969666/jobs/6561686115#step:5:29): in <module>
    class _AwaitEvent:
.nox/unit-3-11/lib/python3.11/site-packages/asynctest/mock.py:433: in _AwaitEvent
    @asyncio.coroutine
E   AttributeError: module 'asyncio' has no attribute 'coroutine'